The electroinitiated polymerization of styrene: Part 1

The electroinitiated polymerization of styrene." Part 1 B. M. T1DSWELL and A. G. DOUGHTY* Electrolysis of a solution of styrene and sodium borofluoride in sulpholane gives rise to low molecular weight polystyrene at the anode. The polymerization occurs by a cationic mechanism initiated by electrolytically produced boron trifluoride co-catalysed by either hydrogen fluoride and/or water. Kinetics in a single compartment cell and a divided compartment cell show differences. Acceleration occurs in the anode compartment of the divided cell with current efficiencies higher than those observed in the single cell. This is ascribed to the presence of an electrolytic termination reaction occurring in the single cell. THE PASSAGE of an electric current through a conducting solution containing an electrolyte and monomer in order to produce polymer has been the subject of several papers over the last few years most of which have been reviewed extensively 1-4. F r o m this work it is obvious that transient species generated at the electrodes initiate polymerization which may be ionic or radical in character or possibly a mixture of both. The aim of the present study was to use an electrolyte capable of producing differen~ species which were able to initiate polymerization by either cationic or anionic mechanisms depending on the supporting solvent. For this purpose sodium borofluoride was used, which by virtue of the BF 4 ions discharged at the anode could possibly give rise to cationic polymerization. Funt and Gray 5 have postulated that discharge of the BF 4 ion during the electrolysis of tetrabutyl a m m o n i u m borofluoride initiates cationic polymerization as a result of the formation of BFa. On the other hand, Na + ions discharged at the cathode could give rise to anionic polymerization by direct or indirect electron transfer through the formation of a radical anion 4. This paper describes the cationic polymerization of styrene whilst Part 2 will describe anionic polymerization. EXPERIMENTAL The solvents used were N,N-dimethyl formamide, DMF, (British Drug Houses Ltd), N,N-dimethyl acetamide, DMA, (BDH Ltd), dimethyl sulphoxide, DMSO, (Koch-Light L t d ) a n d sulpholane, tetrahydrothiophene-l,ldioxide (Koch-Light Ltd). Sulpholane was stirred over sodium hydroxide pellets until no colour developed with concentrated sulphuric acid. Otherwise all solvents were then purified by refluxing over calcium hydride for six hours under an atmosphere of argon. Benzene (10~o volume), previously rigorously dried, was used to azeotrope residual moisture. The remaining * Present address ISR Ltd. Southampton, UK 431

B. M. TIDSWELL AND A. G. DOUGHTY

benzene was then distilled off under argon at atmospheric pressure, and a middle fraction of solvent collected under argon at reduced pressure. The electrolyte, sodium borofluoride (Hopkin and Williams Ltd), was recrystallized from distilled water, filtered and dried under vacuum at 200°C. Argon was purified according to the method described by Funt 6. M o n o m e r s - s t y r e n e (BDH Ltd), acrylonitrile (BDH L t d ) a n d 2-chloroethylvinylether (Koch Light Ltd) were purified according to standard techniques 7.

Apparatus and techniques Electrolysis was carried out at constant temperature and under an inert atmosphere by bubbling purified argon through the reaction mixture using both single and divided cells. In the latter the anode and cathode compartments were separated by a sintered glass filter as described previously 8. The electrodes in all cases were platinum foil (area 1 cm2), and a constant current (1-50 mA) was supplied from a type 1D 50/500 constant current unit (Sandmar Electronic Products Ltd, Manchester). In all cases the initial concentration of NaBF4 was 0.05 M. Because of gaseous products evolved at the electrodes dilatometric techniques could not be used and, instead, the polymers produced were precipitated into ice cold methanol, filtered, washed and dried under vacuum to constant weight. Molecular weights were determined using a Mechrotab vapour pressure osmometer at 37°C with benzene as solvent.

RESULTS AND DISCUSSION

Preliminary experiments showed that, in the divided cell, polymerization of styrene occurs in the cathode compartment when using DMA and to a lesser extent when using D~SO and DMF. On the other hand, with sulpholane as solvent, polymerization occurs almost entirely in the anode compartment. Pre-electrolysis prior to addition of monomer does not alter the position except for a slight increase in yield in the anode compartment when using sulpholane. In view of the different paths of polymerization in DMA and sulpholane copolymerization studies were carried out in these solvents in an attempt to elucidate the general mechanism of polymerization and the results analysed according to the method of Finemann and Ross 9. Comparison, where possible, was made with literature values (Table 1). From this it is reasonable to assume that in sulpholane the reaction proceeds via a cationic mechanism, whilst in DMA the reaction is anionic. The remainder of this paper will be entirely concerned with the polymerization of styrene in sulpholane. The use of discriminant additives to substantiate the cationic mechanism leaves much to be desired, p-Benzoquinone has little effect on the system initially suggesting the absence of a free radical component. The use of diphenyl dipicryl hydrazyl (DPPH) produces some very interesting results. Electrolysis in the presence of styrene and DPPH gives rise to a decrease in yield, the magnitude of which depends on the concentration of DPPH, but 432

ELECTROINITIATED POLYMERIZATION OF STYRENE

no inhibition period is observed. Electrolysis in the absence of monomer shows a decoloration of the DPPH which may be followed quantitatively as a decrease in the 528 m~ absorption peak with time. These results suggest radical activity between possibly BF4. or F. radicals with the DPPH. Table 1 C o p o l y m e r i z a t i o n s t u d i e s Monomersi styrene (MO 2 - c h l o r o e t h y l v i n y l e t h e r (M~) E l e c t r o l y s i s 10 m A at 3 0 ° C in s u l p h o l a n e rl -- 0"9 r2 -- 35 Catalyst : BF3(C2HDzO 2 5 ° C in p y r i d i n e rt ~- 0.11 r2 -- 24 ( B r o w n a n d P e p p e r ) I°

Monomers:

styrene (M1) acrylonitrile (MD Electrolysis 40 mA at 25°C in N,N-dimethyl acetamide rl 0"4 r2 = 14.2 Catalyst: n-butyl lithium, 12°C in iso-octane rl0-2 r2 = 14 ( Z u t t y a n d W e l c h ) 'a

However, if BF8 gas is bubbled into sulpholane a definite 1 : 1 complex is produced in the form of a white crystalline solid which is relatively hydroscopic and unstable. A solution of this complex in sulpholane, giving a concentration of BFs similar to that obtained on electrolysis, produces a similar decoloration of the DPPH which cannot be related to free radical reactivity. In fact BFs (C2H5)20 complex in 1,2-dichloroethane behaves similarly, suggesting that DPPH must be used with care when used as a discriminator for cationic mechanisms. All the systems mentioned were examined by e.s.r., the spectra in all cases showed complete quenching of the radical signal of the DPPH. F r o m these results it is apparent that a complex system develops during electrolysis as detailed in Figure 1. Elemental analysis of the polymer produced allows certain alternatives to be eliminated. No boron or fluorine is found in combination with the polymer, eliminating the possible initiation by BF4 • and F. radicals. However, mass spectroscopic studies of the solvent following electrolysis indicate a parent peak at M/e -- 138 which corresponds to a monofluorinated sulpholane. Qualitative tests using turmeric a2 carried out on the anolyte, in the absence of monomer, following electrolysis showed the presence of BF~. Both calcium chloride gelation la and the decoloration of zirconium alizinarate complex 14 indicate the presence of HF. Analysis of the reaction mixture using the Karl Fisher technique indicated the presence of up to 2mmol/1 moisture which could result in the formation of the BFs(H20)2 complex. Addition of further moisture in the range 1-50 mmol/1 causes a progressive decrease in the initial rate of polymerization. Application of vacuum techniques similar to those developed by Funt 6, in an attempt to eliminate moisture also causes a decrease in the initial rate of polymerization. Similar results were obtained by Clark is in the polymerization of styrene using BFa(H20)2 complex as catalyst but in this case the polymerization was completely arrested on drying. In the present work H F produced during the reaction may also be acting as co-catalyst. 433

B. M. TIDSWELLAND A. G. DOUGHTY

Anode

1

polymer

BF~ - e-

monomer+ H20 and/or HF

monomer L BF2. ~

polymer

BF 3 + F"

,%

BF3/sulpholane complex monomer

I

H'F'+ O / / ~ O F polymer

HF

i F'O ~

F

O

Figure 1 Possible reaction scheme as a result of the anodic discharge of the borofluoride ion. It is interesting to note that electrolysis in the absence of monomer followed by subsequent addition of the anolyte to styrene initiates the polymerization several weeks after the electrolysis was performed suggesting the production of BFa or a complex of BFa. Table 2 illustrates the similarity of the poly-

Table 2 Polystyrene formed by various methods in sulpholane at 30°C Method DPn Conversion(%) Electrolysis for 1 hour at 30 mA in divided cell (anode) together with 2M styrene/NaBFa/sulpholane Electrolysis for 1 hour at 30 mA NaBF4/sulpholane, Anolyte added to 2M styrene in sulpholane BFa/sulpholane complex (0.2102 g) added to 2M styrene in sulpholane, (50 ml)

24

88'1

26

77.4

29

67.3

merization by three methods: electrolysis in the presence of styrene, electrolysis in the absence of m o n o m e r followed by subsequent addition of the anolyte to monomer, and finally polymerization using the BFz/sulpholane complex dissolved in sulpholane. These results demonstrate that on electrolysis of a sulpholane/NaBF4/ styrene solution, the styrene polymerizes cationically as a result of the formation of BFa, the reaction being catalysed by H~O and/or HF. Figure 2 shows conversion/time curves obtained at several currents using a single cell. Values of the initial slopes of these curves, expressed as initial rate plotted against current, show a linear relationship with a slope of unity 434

ELECTROINITIATED POLYMERIZATION OF STYRENE

indicating a first order dependence on current (Figure 3). At the lower currents slight inhibition is evident which may be due to the electrolysis of impurities. Figure 4, illustrates conversion/time curves over a ten fold range of monomer concentration whilst maintaining the current constant at 10 mA.

E

'~

-10

®

g 0mA

~

~"

s

o

lOmA

g (j

Q.

"6 -q .~_ >-

0 0

1

2

3 4 T i m e (h)

= 5

0

Figure 2 The formation of polystyrene with time at various currents for 2M styrene in sulpholane/NaBF4 in the single cell

Slight upward curvature of the plots is evident at the higher concentrations indicating possible slight acceleration of the reaction. The initial slopes of these curves again expressed as initial rates plotted against the square of the monomer concentration give a linear relationship as shown in Figure 5, indicating a second order dependence on monomer concentration. Kinetic analysis of data obtained from protracted electrolysis to higher conversions substantiated this hypothesis. When using the divided cell with systems otherwise identical to those used previously much higher conversions are observed together with considerable acceleration at all currents increasing with increasing current (Figure 6). However, taking the initial slopes of these curves as being representative of a steady state condition, when plotted against current give a linear first order relationship (Figure 3). Again when the monomel concentration is varied, at a constant current, acceleration occurs and increases with increasing monomer concentration (Figure 7). The initial slopes of these curves are proportional to the square of the monomer concentration as in the case of the single cell (Figure 5). In the case of the divided cell post-polymerization effects are observed, polymerization continuing after cessation of electrolysis. The magnitude of the effect depending not only on the quantity of current passed but on the rate of electrolysis. The effect is greater, for example, after passing 30 mA for one hour than l0 mA for 3 hours (Figure 8). From these results it would appear that in the anode compartment of the divided cell there is a continuous production of active catalyst which gives rise to an acceleration of the rate of polymerization and which is capable of 435

B. M. TIDSWELL AND A. G. DOUGHTY

3 Divided cett (anode)

/

~o

L -6 E o

~gie ceil

-6 c

0

~

0

10

20

30

t,0

50

Current. (mA) Figure 3 D e p e n d e n c e of initial rate o f electropolymerization on the c u r r e n t for 2 M styrene in s u l p h o l a n e / N a B F 4

/

0-06 Z

8

Z 00~

/

/.oj

_

/

~o. 0.02

"a ~"

2.0 O' 0

v

-

2

3

4

5

6

T i m e (h)

Figure 4 T h e f o r m a t i o n of polystyrene with time at various initial m o n o m e r c o n c e n t r a t i o n s at 10 m A in s u l p h o l a n e / N a B F 4 in the single cell 436

ELECTROINITIATED POLYMERIZATIOM OF STYRENE

some further initiation after electrolysis has ceased. On the other hand, in the single cell, destruction of a similar catalyst is brought about by the presence in the same compartment of either the cathode or cathodically produced products.

Divided

ceil.(anode]

10 mA

-o~ 0

I

I

I

B

16

2/+

r.styrene] 2 (mo[ t-1 )2 Figure 5 D e p e n d e n c e of initial rate of electropolymerization of styrene on the s q u a r e o f the initial m o n o m e r c o n c e n t r a t i o n in sulpholane/NaBF4

The degree of polymerization of the polymers produced at low conversions is relatively low even over a tenfold change in monomer concentration:

single cell 0.5-5.0 M styrene, DPn -- 13-9 anode compartment divided cell 0.5-5.0 M styrene, DPn = 20-25 It would appear therefore that considerable chain transfer to monomer is taking place as is indicated by the presence, in the i.r. spectra of the polymer, of the weak absorption band at 965 cm -1 which has been assigned to an out of plane deformation of a trans double bond at the end of a polymer chain 16. Application of the Mayo equation 17 to results shows a linear relationship between 1/DPn and I/(monomer) in the case of the divided cell from which values of km/kp = 0"034 and kt/kp = 0.0175 are obtained (kin, k~, kt being the rate constants for transfer to monomer, propagation and spontaneous 437 2D

B. M. T I D S W E L L A N D A. G . D O U G H T Y

termination, respectively). These are similar to other values reported for styrene, za In the case of the single cell the variation of molecular weight with monomer concentration is difficult to interpret. This may be due to the presence of

0'151

75

~0mA

c

O)

O.lO

50

20mA C: c

.o 0

"6

O

._~ 0-05 >-

25

r25 mA

5 mA

0 0

~ 1

x----~ 0 2 3 /, Time (h) Figure 6 The formation of polystyrene with time at various currents for 2M styrene in sulpholane/NaBF4 at the anode of the divided cell

important termination mechanisms other than transfer to monomer. Figure 9 shows how, after prolonged electrolysis, the percentage conversion is quite love in the case of the single cell whilst in the divided cell the conversion calculated for the half cell is quite high, and approaches 100 ~o at the higher monomer concentrations. From a knowledge of the yield of polymer and the number of Faradays passed the current efficiency (CE), the number of moles of monomer polymerized per Faraday, can be calculated. Again in the case of the single cell the efficiency is low whilst in the case of the divided cell the efficiency approaches the theoretical, especially at higher monomer concentration. By dividing the current efficiency by the average degree of polymerization of the polymer the number of molecules of polymer produced per electrical 438

ELECTROIN1TIATED POLYMERIZATION OF STYRENE

s0Ml

0.31

r-

4"ON

"~0"2 e-

o

"6 -o

.~_ >-

3'ON

0"1 2-OM O'5M O

~

0

1

2

3 Time

/~

(h)

Figure 7 The formation of polystyrene with time at various initial monomer concentrations at 5 mA in sulpholane/NaBF4 at the anode of the divided cell. 10

Oq4. E

o

O

O

i m

i

50

"6 0'12

o

50,-

oqo

o i/1

~0g~ "6 ~o

O

w

1 2

I 3

O'OB'

30 I 1

Time (h)

Figure 8 Post-polymerization at the anode of the divided cell for 2M styrene in sulpholanel NaBF4: ©, 30mA for l ' 0 h ; m , 25mA for l-5h; A , 20mA for 2.5h, O , 10mA for 3"0h 439

B. M. TIDSWELL AND A. G. DOUGHTY

Table 3 Electropolymerization of styrene sulpholane/NaBF4 Styrene (mol)

YieM (g)

Conversion Current (%) efficiency (tool/Farad)

Molecules polymer/ electrical event

J

Single cell (7h, 50 mA, DPn ~ 12) 0'5 0"0531 1.02 1'0 0.4441 4"3 2.0 2"4535 11 '8 3-0 9.6443 30.9 4'0 18'1608 43"6 5.0 25.7524 49.5

0-04 0"3 1.8 7-1 13.4 18.9

0.003 0.03 0.1 0.6 1.1 1'6

J

Divided cell (anode compartment) (lh, 30 mA, DPn = 25)

0"5 1"0 2"0 3'0 4'0 5.0

1.3002 3"4566 9.1731 14"2106 19"3545 25.4001

49"9 66"4 88"1 91"0 93'0 97.3

11"0 29.7 78"7 121 "9 166"1 218-1

0"4 1"2 3'2 4.9 6"6 8'7

event is obtained. As can be seen in Table 3, in the case of the divided cell at the highest concentration some eight polymer molecules are produced for each individual electrical event indicating considerable chain transfer. At the lowest concentration, there appears to be no transfer, the initiation is apparently quite efficient, approaching 50 ~o. In the case of the single cell, at the lowest concentration, the value is extremely low increasing to 1.58 at the highest concentration. In view of the low value at the low concentration it would appear that at the highest concentration there could be some chain transfers taking place. The lower yield, current efficiencies and molecular weights of the polymers produced in the single cell may be due to the presence of additional termination steps which are absent in the divided cell. The presence of the cathode in the single cell could either remove growing carbonium ions by electrolysis or produce negatively charged species capable of reaction with the propagating carbonium ions. In the former case proton expulsion from the chain may occur: ÷ ~-CH2--CoH~ + e - ~ , C H ~ C H C o H 5 + HIn the latter case reaction of cathodically discharged sodium with the sulpholane may lead to products capable of chain termination. Analysis for sulphur in the polymer indicates that at the most only one sulphur atom per 12 polymer chains is available. It is apparent from these results that some type of electrolytic termination process is active in the single cell. Thus DPn may be expressed as: DPn = R~/(Rm + Rt + Re)

(1)

where .R~, Rm, Rt and Re are the rates of propagation, transfer to monomer, spontaneous termination and electrolyte termination, respectively; 440

ELECTROINITIATED POLYMERIZATION OF STYRENE

Single cell

LL

60

. ,~

,or,00" o

/ .

monomer

/

_ - t,O

/

g .~t0

%

-20 ~c

~o 0 a

1

2

Initio[

3

,

i

¢

5

o

styrene conch (mot L-1)

240

200

I00

~6c

Bo

~oo

so g ~

ia

0 b

2

1

Initio[

3

styrene concn

4

5

( m o L t -+ )

Figure 9 Variation in current efficiency and percentage conversion with initial monomer concentration in sulpholane/NaBF4 where Rp = kp[Mn + ] . [M]

Rm =

k m [ M n + ] . [M]

(2) (3)

Rt = kt[Mn +]

(4)

Re =

(5)

k e [ M n + ] . [1]

where [M] ~ concentration o f m o n o m e r [I] = current expressed as F a r a d a y per litre 1

km

kt

ke[I]

thus DPn -- kp + kp[M--~] + kp[M] Thus a linear relationship should exist between 1/DPn and [I] f r o m the slope o f which ke/kp m a y be evaluated. In order to substantiate this the total 441

B. M. T I D S W E L L AND A. G. D O U G H T Y

amount of polymer produced, during electrolysis, of 2M styrene solutions at 30°C and at various currents, was isolated and the value of 1/D'Pn plotted against [I] (Figure 10). In the single cell 1/DPn can be seen to increase with increasing current giving a value of ke/k~ of 130 × 10 -2 indicating considerSingle celt

12

10 % X

-9. I

I

I

I

2

t.,.

6

8

10

[ I ] (F t-1)xlO =

Divided celt (anode)

6 X

I 0

b

I

1

[I] ( F t "~) xIO 2

Figure 10 Effect of current on molecular weight of polystyrene formed during the electrolysis of sulpholane/NaBF4 !

able electrolytic termination. In the case of the divided cell 1/DPn is independent of current indicating the apparent absence of electrolytic termination in this case as expected. It would appear from this evidence that polymerization is catalysed by anodically generated boron trifluoride and co-catalysed by either water or hydrogen fluoride (Figure 1) and that the concentration of the active catalyst is proportional to the quantity of current passed. 442

ELECTROINITIATED POLYMERIZATION OF STYRENE I f one a s s u m e s t h a t s t e a d y state c o n d i t i o n s apply d u r i n g the initial stages o f p o l y m e r i z a t i o n then, for the single cell, the rate e q u a t i o n m a y be written as

k~k~[ M]2[ I ] Rp = kt ÷ kin[M] q- he[I]

(7)

whilst in the divided cell at the a n o d e under similar c o n d i t i o n s :

kpk~ [M ]2[I ] Rp -- kt -~- km[M] The m a i n difference being t h a t in the single cell there is a considerable electrolytic t e r m i n a t i o n step which limits the m o l e c u l a r weight a n d the p e r c e n t a g e conversions. It is difficult to u n d e r s t a n d the reason why s e p a r a t i o n o f the two electrodes by a single glass filter is sufficient to cause such a considerable effect and further w o r k is in progress in an a t t e m p t to explain this p h e n o m e n a .

ACKNOWLEDGEMENT T h e a u t h o r s wish to a c k n o w l e d g e with g r a t i t u d e the g r a n t o f a University o f B r a d f o r d research scholarship t o one o f t h e m ( A . G . D . ) .

School o f Polymer Science, The University, Bradford 7, Yorkshire, UK

(Received 16 November 1970)

REFERENCES 1 Tidswell, B. M. Rep. Prog. Appl. Chem. 1968, 53, 516 2 Funt, B. L. Macromolecular Rev. 1966, 1, 35 3 Asahara, T. and Sen, M. J. Synth. Org. Chem., Japan 1967, 25, 719 4 Yamazaki, N. Adv. in Polym. Sci. 1969, 6, 377 5 Funt, B. L. and Gray, D. G. J. Maeromol. Chem. 1966, 4, 1 6 Funt, B. L. Bhadani, S. and Richardson, D. Can. J. Chem. 1966, 44, 711 7 Elliott, V. R. in Macromoleeular Synthesis 1966, 2, 30 and 86, Wiley, New York, 1966 8 Tidswell, B. M. S.C.I. Monograph 1966, 20, 129 9 Finemann, M. and Ross, S. D. J. Polym. Sci. 1950, 5, 269 10 Brown, G. R. and Pepper, D. C.J. Chem. Soe. 1963, p 5930 1l Zutty, N. L. and Welch, F. T. J. Polym. Sci. 1960, 43, 445 12 Kolthoff, I. M. and Elving, P. J. 'Treatise on Analytical Chemistry: Part 2. Analytical Chemistry of the Elements,' vol 7, p 290, Interscience, New York, 1961 13 Simons, J. H., 'Fluorine Chemistry', vol 2, p 62, Academic Press, New York, 1954 14 Kline, G. M., 'Analytical Chemistry of Polymers: Part 3. Identification and Chemical Analysis,' p 61, Interscience, New York, 1962 15 Clark, D. in 'Cationic Polymerisation and Related Complexes' (Editor Plesch, P. H.) p 69, Heifer, Cambridge, 1953 16 Rao, C. N. R., 'Chemical Applications of Infra-red Spectroscopy', p 147, Academic Press, New York, 1963 17 Allen, P. E. M., in 'The Chemistry of Cationic Polymerisation' (Editor, Plesch, P. H.), p 122, Pergamon, Oxford, 1963 18 Mathieson, A. R., in 'The Chemistry of Cationic Polymerisation' (Editor, Plesch, P. H.), p 286,Pergamon, Oxford, 1963 443